1. Field of the Invention
This invention relates to aqueous electrolyte formulations based on alkanesulfonic acids. These electrolyte formulations are intended for the electrodeposition of copper, especially on electronic devices.
2. Prior Art
Electrolytic copper plating is a process which deposits a layer of copper on metallic or non-metallic substrates using an external electric current. Commercial copper plating solutions include copper sulfate, copper pyrophosphate, copper fluorborate and copper cyanide. Copper sulfate and copper fluorborate solutions are typically used at medium to high current densities whereas copper pyrophosphate and copper cyanide solutions are used to deposit copper at low to medium current densities. Because of the health concerns associated with handling cyanide salts and/or fluorboric acid and because of the waste-treatment concerns with cyanide, fluoborate and pyrophosphate based systems, the most widely used commercial copper plating electrolyte is based on copper sulfate and sulfuric acid.
Copper sulfate based plating solutions are used to deposit a copper coating on various substrates such as printed circuit boards, automobile parts and household fixtures. The copper ion concentration in typical solutions varies from about 10 grams per liter to about 75 grams per liter. The sulfuric acid concentration may vary from about 10 grams per liter to about 300 grams per liter. Copper solutions intended for the plating of electronic components usually employ low copper metal concentrations and high free acid concentrations.
The use of alkanesulfonic acids in electroplating has been described previously. Proell, W. A. in U.S. Pat. No. 2,525,942 claims the use of alkanesulfonic acid electrolytes in numerous types of electroplating. For the most part, Proell""s formulations employed mixed alkanesulfonic acids. In U.S. Pat. No. 2,525,942 Proell made specific claims for lead, nickel, cadmium, silver and zinc. In another U.S. Pat. No. 2,525,943, Proell specifically claims the use of alkanesulfonic acid based electrolytes in copper electroplating, but again only mixed alkanesulfonic acids were used and the exact compositions of the plating formulations were not disclosed. In a separate publication (Proell, W. A.; Faust, C. L.; Agruss, B.; Combs, E. L.; The Monthly Review of the American Electroplaters Society 1947, 34, 541-9) Proell describes preferred formulations for copper plating from mixed alkanesulfonic acid based electrolytes. Dahms, W. and Wunderlich, C. in German Patent No. 4,338,148 described an MSA based copper plating system which incorporates organic sulfur compounds as additives. In a Chinese publication (Jiqing, Cai; Diandu Yu Huanbao 1995, 15(2), 20-2) the author shows some of the benefits of using MSA based acid copper plating formulations. The greatest benefit claimed by Jiqing was a superior surface cleaning and etching prior to the actual plating step. In U.S. Pat. No. 5,051,154 (Bernards, R. F.; Fisher, G.; Sonnenberg, W.; Cerwonka, E. J.; Fisher S.), there are described surface active additives for copper plating with minor mention of MSA as one of a number of possible electrolytes. Andricacos, P. C., Chang, I. C., Hariklia, D. and Horkans, J. in U.S. Pat. No. 5,385,661 discuss a process which allows for the electrodeposition of Cu alloys containing small amounts of tin and lead via under-potential deposition. The Andricacos patent claims that MSA is exceptionally well suited for promoting the proper functioning of this type of process, owing mostly to the weakly complexing nature of MSA/OMs. A paper on this subject (J. Electrochem. Soc.; 1995; 142(7); 2244-2249) was also published.
Increased densities of transistors on silicon wafers has required the development of new metallization technologies for the plating of fine-line structures. Until recently, aluminum was used as the metal interconnect, but recent developments in integrated circuit technology have shown that copper is the preferred metal for interconnects in electronic components. Copper deposited from electroplating solutions has been shown to be the most economical way to meet the needs of the modern interconnect industry.
In the processing of semiconductor devices, several metallization steps are required. This type of metallization has previously been accomplished with vapor deposition techniques. Recently, electroplating techniques capable of metallizing semiconductor components have been developed. Prior to copper electrodeposition, a copper seed layer which acts as a catalyst is deposited on the silicon wafer. This copper seed layer is about 100-500 nm thick. The semiconductor surface is etched with numerous sub-micron dimensioned interconnecting trenches, and copper is next electroplated onto the seed layer with the filling of these trenches from the bottom upward. Because of the high free acid level used in optimized copper sulfate based plating solutions, about 150-200 gram per liter, the copper seed layer is oftentimes attacked and a significant portion of it may dissolve prior to the initiation of the copper electroplating.
Because of the need to deposit a smooth and fine-grained copper deposit, organic grain refining additives are always added to the copper plating solution. For example, Martin, S. in U.S. Pat. No. 5,328,589 describes the use of surface active materials including alcohol alkoxylates and nonionic surfactants as additives in copper plating baths. Martin, S. also discloses in U.S. Pat. No. 5,730,854 the use of alkoxylated dimercaptans as additives in copper plating baths. These additives inhibit copper deposition at high current densities resulting in a continuous and smooth deposit. Such additives are consumed during the deposition process, and a portion of these additives may be incorporated into the copper deposit. The co-deposition of organic additives in a copper deposit may affect the electrical conductivity of the deposit, and frequent analysis is necessary to ensure a constant organic additive concentration in the copper plating solution.
It would be useful in connection with the various processes that deposit copper into trenches and vias on ceramic substrates to have a copper solution that can operate at a lower free acid concentration than existing optimized copper sulfate based solutions. Such solutions will be less corrosive to the copper catalytic seed layer, and they will require a lesser quantity of additives than current copper sulfate based solutions. In addition, these low free alkanesulfonic acid based solutions will allow the deposition of a smoother coating.